The primary objective of the study is to compare disease-free survival (DFS) of patients with high-risk cutaneous squamous cell carcinoma (CSCC) treated with adjuvant cemiplimab, versus those treated with placebo, after surgery and radiation therapy (RT).

The secondary objectives of the study are:

• To compare the overall survival (OS) of high-risk CSCC patients treated with adjuvant cemiplimab, versus those treated with placebo, after surgery and RT
• To compare the effect of adjuvant cemiplimab with that of placebo on patients' freedom from locoregional recurrence (FFLRR) after surgery and RT
• To compare the effect of adjuvant cemiplimab with that of placebo on patients' freedom from distant recurrence (FFDR) after surgery and RT
• To compare the effect of adjuvant cemiplimab with that of placebo on the cumulative incidence of second primary CSCC tumors (SPTs) after surgery and RT
• To evaluate the safety of adjuvant cemiplimab and that of placebo in high-risk CSCC patients after surgery and RT
• To assess cemiplimab pharmacokinetics and immunogenicity in human serum

Cemiplimab is a monoclonal antibody. Antibodies are proteins naturally found in the blood. A monoclonal antibody is a special antibody that is manufactured as a medication to target specific proteins in the body that may be involved in cancer. Cemiplimab works by blocking programmed death-1 (PD-1), a cell receptor on immune cells. By blocking PD-1, it is expected that the immune cells will attack cancer cells.

The study is being conducted in participants that have had surgery and radiation therapy for a type of skin cancer called cutaneous squamous cell cancer, and who have a risk that this cancer may come back.

The main purpose of the study is to determine if cemiplimab will prevent cutaneous squamous cell cancer (CSCC) from returning after surgery and radiation. Currently, we know that certain types of CSCC have a high chance of coming back after surgery and radiation. At this time, there is no approved treatment to give patients after surgery and radiation to prevent high-risk CSCC from coming back. We are investigating if the addition of cemiplimab will decrease the chance of these high-risk cancers coming back.

The study will also investigate if cemiplimab may help participants live for longer.

The study has two parts. Part 1: participants will receive every 3 weeks via intravenous infusion (drip into a vein) either cemiplimab (study drug) or placebo (similar to the study drug but without active medicine). After 12 weeks of treatment, cemiplimab or placebo will be given every 6 weeks. Part 1 of the study includes a screening period (up to 28 days), a treatment period (approximately 48 weeks), an end of treatment visit (approximately 30 days after completion of study drug treatment period) and a post-treatment follow-up period (approximately up to 5 years or until skin cancer returns or the study ends).

Part 2 of the study is optional and provides the participant with the option to receive cemiplimab if the cancer comes back if the participant was initially receiving placebo. There is no placebo in Part 2 of the study. If the cancer comes back, the study doctor will discuss with participants if they are eligible to participate in the optional part 2 of the study.

Part 2 of the study includes a brief screening period, a treatment period (approximately 96 weeks) and an end of treatment visit (approximately 30 days after the completion of the study drug treatment period).

Key Inclusion Criteria:

• For Japan only, men and women ≥21 years old
• Patient with resection of pathologically confirmed CSCC (primary CSCC lesion only, or primary CSCC with nodal involvement, or CSCC nodal metastasis with known primary CSCC lesion previously treated within the draining lymph node echelon), with macroscopic gross resection of all disease
• High risk CSCC, as defined in the protocol
• Completion of curative intent post-operative radiation therapy (RT) within 2 to 10 weeks of randomization
• Eastern Cooperative Oncology Group performance status (ECOG PS) ≤1
• Adequate hepatic, renal, and bone marrow function as defined in the protocol

Key Exclusion Criteria:

• Squamous cell carcinomas (SCCs) arising in non-cutaneous sites as defined in the protocol
• Concurrent malignancy other than localized CSCC and/or history of malignancy other than localized CSCC within 3 years of date of randomization as defined in the protocol
• Patients with hematologic malignancies (note: patients with chronic lymphocytic leukemia (CLL) are not excluded if they have not required systemic therapy for CLL within 6 months of enrollment)
• Patients with history of distantly metastatic CSCC (visceral or distant nodal), unless the disease-free interval is at least 3 years (regional nodal involvement of disease in draining lymph node basin that was resected and radiated prior to enrollment will not be exclusionary)
• Ongoing or recent (within 5 years of randomization date) evidence of significant autoimmune disease that required treatment with systemic immunosuppressive treatments, which may suggest risk for immune-related adverse events (irAEs). The following are not exclusionary: vitiligo, childhood asthma that has resolved, type 1 diabetes, residual hypothyroidism that required only hormone replacement, or psoriasis that does not require systemic treatment.
• Has had prior systemic anti-cancer immunotherapy for CSCC

Note: Other protocol defined Inclusion/Exclusion criteria apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf